The present invention relates to a method of laminating a gas permeable (breathable) sheet material such as a non-woven fabric, a woven fabric, a knit, a web, a fibrous mat, a film material and so on. The present invention particularly relates to a method of heat laminating such gas permeable sheet materials together into an integrated laminate form while maintaining their gas permeability.
The present invention also relates to a method of laminating of a sheet material having gas permeability which is used in various industrial fields. The sheet material is used as, for example, a garment material and a sanitary material, a packaging material, various filter materials, a building material, a substrate for an electret material and so on.
In a method of laminating a plurality of gas permeable sheet materials into an integral form which has been employed for many years, the sheet material comprising a thermoplastic material are heated to its melting point or higher using a heating roll, and a pressure is vertically applied to the sheet materials using, for example, a nip roll at the same time of or immediately after heating the sheet materials along a thickness direction of the materials so that adhesion through the melted thermoplastic material is accelerated.
However, there is an undesired effect in the above method that the thermoplastic material is rolled by the nip roll, whereby many gas permeable pores through the sheet material are unnecessarily plugged so that original gas permeability of the sheet material cannot be maintained. With such a method of the prior art, a gas permeability retention ratio is said to be at most about 65%.
In order to overcome the above problem, various methods have been proposed to laminate a plurality of the gas permeable materials together into an integral form while maintaining their gas permeability. For example, the following methods can be exemplified:
1) A fusible adhesive is disposed on a gas permeable material in the form of dots or lines, and then other gas permeable material is laminated on the gas permeable material;
2) A tape or film having thermoplasticity is disposed on a gas permeable material with a given separation, and then other gas permeable material is disposed on the gas permeable material so as to heat laminate them (see, for example, Japanese Patent Kokai Publication No. 64433/1992 and Japanese Patent Kokai Publication No. 249739/1987;
3) A thermoplastic synthetic resin is spread on a gas permeable material or a thermoplastic synthetic resin emulsion is applied to a gas permeable material, then other gas permeable material is placed on the gas permeable material, and the both materials are heated to melt the synthetic resin so that the gas permeable materials are laminated together (see, for example, Japanese Patent Kokai Publication No. 179545/1992 and Japanese Patent Kokoku Publication No. 14889/1990);
4) Superposed gas permeable materials are partially heat-fused using an embossing roll (see, for example, Japanese Patent Kokai Publication No. 194912/1989);
5) A non-woven fabric comprising a conjugated fiber (namely, a composite fiber comprising a core/heat-fusible sheath structure) is superposed on a gas permeable material, and they are heat laminated with a minimum pressure applied by a felt calendaring roll (see, for example, Japanese Patent Kokai Publication No. 122752/1987); and
6) A gap (a clearance) between heat laminating rolls, a lamination temperature and a roll rotating speed are adjusted to partially heat-fuse gas permeable materials so as to achieve a fused area ratio of 5 to 95% (see, for example, Japanese Patent Kokai Publication No. 293008/1991).
In the methods of the above 1) to 4), since a surface of the gas permeable material is partially plugged, the gas permeability of the material is decreased so that the gas permeability inherent in the material cannot be fully maintained. In addition, even when the felt calendaring roll is used with the minimum pressure as in the process of 5), the gas permeability is decreased by at least 30%.
Contrary to those methods, since the method of 6) adjusts the gap between the heat laminating rolls and the like to control the heat-fused area in a range of 5 to 95%, almost no decrease of the gas permeability (which is measured as a pressure drop increase) occurs. However, the adjustment of the gap is so sensitive that only about 0.2 mm change of the gap makes it impossible to maintain the gas permeability. In addition, the sensitive adjustment of the gap is required whenever the material or a weight (METSUKE) of the gas permeable material is changed. Further, a commercially available gas permeable material such as a non-woven fabric varies in its thickness. It is, therefore, very difficult to stably apply a pressure to keep a constant gap for the lamination. When a rubber roll is used as a press roll in order to accommodate such variations in thickness, there occurs a problem that the gap cannot be controlled due to thermal expansion of the rubber.
It is an object of the present invention to overcome the problems as described above, that is, the decrease of the gas permeability and the unstable production of the laminate in the methods of the prior art, and to provide a method of laminating gas permeable materials together while maintaining their gas permeability which process is not affected by ambient factors such as a weight or a thickness of the gas permeable material and which process is applicable for the convenient mass production of the laminate.
As a means to achieve the above object, the present invention provides a method of laminating at least two gas permeable sheet materials characterized in that at least a portion of at least a surface region of at least one of said sheet materials which are adjacent to one another comprises a thermoplastic material, said sheet materials are superposed and then said sheet materials are laminated without direct pressure application to them along a thickness direction of said sheet materials on and after contact with a heating means which heats said thermoplastic material to or above its melting point so as to melt said thermoplastic material whereby said sheet materials are laminated to one another.
Particularly, according to the present invention, there is provided a method in which at least two adjacent gas permeable sheet materials such as a cloth or a non-woven fabric or a film material are laminated without degradation of the gas permeability of the sheet materials characterized in that at least one of said adjacent gas permeable sheet materials comprises a thermoplastic material in at least a portion of at least its surface region, the sheet materials are superposed and then, on and immediately after contact with a heating means (for example, a heating roll or a heating plate) which heats said thermoplastic material to or above its melting point, a pressure is applied to said sheet materials neither positively nor directly along a thickness direction of said sheet materials by, for example, a roll such as a nip roll.